Which types are mutable and immutable in the Google Go Language?

Question

In Google Go, I read that Strings are immutable, ok but are int's? What about other types? As a slightly older programmer I prefer mutability even though I know the benefits of immutability, I prefer to live dangerously.

Know what types are mutable or immutable would be very helpful.

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Update, what I am mostly concerned about is the practical issues depending upon the type being mutable or immutable. As in the typical example in Java, if you create a String in a loop and loop for 10,000 times, you will get 10,000 String's created which are then later garbage collected. This has actually been a serious issue in a project in a company I worked at.

The the question is, does Go's Immutability in some cases cause the same problem?

It affects how you should treat the var. (or I assume it does).

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Update again, I am also concerned about other practical concerns. Knowing that something is immutable means that I can write code which is parallel and updates to one reference of the object should not update the other references. However sometimes I wish to do dangerous things, I want mutability.

These are consequences of mutability vs immutability and affect how I can write the code.

Answer 1

Don't worry -- Go will let you shoot yourself in the foot if you really want to :-)

Go is not like Erlang, which might be what you are getting at with the question.

x := 1
x = 2

allocates one variable, x, with a value of 1, then reassigns it to 2 -- no additional memory is allocated here.

As you note, strings are immutable, so doing a string manipulation can result in making copies. If you find that you want to do in-place modifications to character data, you'll probably want to operate on variables of []byte via the bytes package.

Russ Cox's post about this should answer most of your questions about basic data structures: http://research.swtch.com/2009/11/go-data-structures.html

As other commenters noted, you'll want to look at the value semantics of Go functions -- they might be a little surprising at first.

If you have the following function:

func (t MyType) myFunc() {
    // do something to set a field in t
}

and you call in your code

myVar.myFunc()

you might be surprised to see that this doesn't do what you want because the t that is seen in myFunc() is really a copy of myVar.

But, the following will work:

func (t *myType) myFunc() {
    // do something to set a field in t
}

because the function has a copy of the pointer and can access the underlying structure via that pointer.